[chore](trx-rs): add OpenRC service for act_runner on Alpine

The runner host is Alpine (OpenRC, no systemd). Add an OpenRC init script
for act_runner (supervise-daemon, depends on docker) plus a conf.d
example for running one instance per project, and rewrite the runner
section of the README with Alpine setup steps (apk docker, dedicated user
in the docker group, register, service install).

Assisted-By: Claude Code (claude-opus-4)
Claude-Session: https://claude.ai/code/session_01NFpGtGTWUEYXLwZeZs2RAV
Signed-off-by: Stan Grams <sjg@haxx.space>

container/act_runner.openrc

@@ -0,0 +1,44 @@
+#!/sbin/openrc-run
+# SPDX-FileCopyrightText: 2026 Stan Grams <sjg@haxx.space>
+# SPDX-License-Identifier: GPL-2.0-or-later
+#
+# OpenRC service for a Gitea act_runner (Docker executor) on Alpine.
+#
+# Install as /etc/init.d/act_runner (chmod +x). Single instance uses
+# /etc/act_runner/config.yaml. For one runner per project, symlink this script
+# and add a matching conf.d file:
+#
+#   ln -s act_runner /etc/init.d/act_runner.trx-rs
+#   cp container/act_runner.confd.example /etc/conf.d/act_runner.trx-rs
+#   $EDITOR /etc/conf.d/act_runner.trx-rs      # set runner_dir / runner_config
+#   rc-update add act_runner.trx-rs default
+#   rc-service act_runner.trx-rs start
+
+description="Gitea Actions runner"
+
+: "${runner_user:=act}"
+: "${runner_dir:=/var/lib/act_runner}"
+: "${runner_config:=/etc/act_runner/config.yaml}"
+
+command="/usr/local/bin/act_runner"
+command_args="daemon --config ${runner_config}"
+# No group given, so supplementary groups (incl. docker) are initialised.
+command_user="${runner_user}"
+directory="${runner_dir}"
+
+supervisor="supervise-daemon"
+respawn_delay=5
+respawn_max=0
+pidfile="/run/${RC_SVCNAME}.pid"
+output_log="/var/log/${RC_SVCNAME}.log"
+error_log="/var/log/${RC_SVCNAME}.log"
+
+depend() {
+	need docker
+	use net dns
+}
+
+start_pre() {
+	checkpath -d -m 0750 -o "${runner_user}" "${runner_dir}"
+	checkpath -f -m 0640 -o "${runner_user}" "${output_log}"
+}